# 6. Speed-Time Graphs

# Success Criteria

Your learning has been successful if you can do the following:

Define acceleration and its units.

Calculate acceleration from speed and time measurements.

# Vocabulary

Learn these so you can communicate this concept well.

Average speed: Calculated by the total distance travelled divided by the total time taken for the journey.

Gradient / slope: How steep a line on a graph is.

Speed: How fast an object is moving.

Velocity: Similar to speed, but also tells us the direction in which an object is moving. It includes both speed and direction of motion.

Do Now: Find some space in SciPAD pg 22

Write out the completed sentence:

Acceleration of an object can be found using this formula: a = Δ_/Δ_, where Δ (delta) means ___________.

The change in speed Δv is the _____ speed minus the _____ initial speed. The change in time Δt is the ______ _______.

Find some space on page 15 of your PESS1.2 SciPAD,

and answer the following questions:

What does each letter in CUTLASS stand for?

## Speed-Time Graphs

If you can measure the speed of a moving object at regular time intervals, a SPEED-TIME GRAPH for that object can be drawn.

A speed-time graph for a car ride can be drawn by taking a SPEEDOMETER reading at regular time intervals. The data below shows the results of such an experiment.

A LINE GRAPH (time on the horizontal axis and speed on the vertical axis) drawn for this data gives you the speed-time graph.

This graph to the left shows that the car is speeding up or ACCELERATING from the start until it reaches the speed of 50 m/s. The car then keeps the same speed for the rest of its journey (CONSTANT SPEED).

## What do the Shapes of Speed-Time Graphs Tell Us

(Come back to this later, after you've learned about Newton's Laws of Motion)

Whenever there is a change in velocity, either due to a change in speed or a change in direction, there will acceleration.

If the net force that causes this acceleration is constant, then acceleration will be constant. Acceleration will not be constant if the net force is not constant.